An electric pump can fill a tank in 3 hours. Because of a leak in the tank, it took 3 hours 30 min to fill the tank. The leak can drain out all the water of the tank in :

A. 10 hours 30 min   
B. 21 hours
C. 12 hours   
D. 24 hours
Answer:     B

Explanation:     

Work done by the leak in 1 hour = (1/3) - (2/7) = 1/21 .
Leak will empty the tank in 21 hours.